Research project: Mechanics of landslides

Currently Active:

Yes

Large-scale landslides can seriously threaten human life and infrastructure over extensive areas, by rapidly moving substantial volumes of material and even by causing catastrophic tsunamis. Better understanding their mechanics is key for predicting such events and protecting ourselves from their consequences.

Project Overview

Large-scale terrestrial and submarine landslides pose a serious threat to human life and infrastructure as they can mobilise a substantial volume of material and affect extensive areas, or even trigger catastrophic tsunamis. A remarkable aspect of submarine landslides in particular is that they can occur on very shallow slopes, often comparable to the slope of a football pitch; such low gradients are almost always stable on land. Also, although some landslides continue to slowly creep over many years, others may develop into catastrophic events by suddenly accelerating to high velocities.

An often quoted example of a catastrophic submarine landslide is the massive Storegga slide off the coast of Norway, which mobilised a volume of over 3,000 cubic kilometres, had a run-out of 800 km and generated a tsunami that was 3-6m high when it reached the east coast of Scotland. Another example of a catastrophic landslide is the 1963 Vaiont slide in Italy; this was a case of a creeping, but otherwise considered under control, landslide that suddenly accelerated into a reservoir, causing a wave that overtopped the dam and killed 2,600 people downstream. A more recent example is the submarine landslide that caused the 1998 Sissano tsunami, claiming 2,000 lives and leaving 12,000 people homeless.

How can submarine landslides initiate in very shallow slopes?

In collaboration with the National Oceanography Centre at Southampton we investigate particular mechanisms that may “precondition” a slope for failure and, individually or in combination, make the initiation of a landslide possible. These include:

Rapid sediment deposition. If fine-grained sediments accumulate relatively quickly (in geological terms) on the continental shelf, there may not be enough time for the water in them to escape and the excess pore water pressure will make the sediment more prone to instability.

Destructuring of marine sediments. If the underlying sediment was deposited under conditions that promoted an “open” structuring of its particles, continuing deposition at a higher rate could cause these structures to suddenly collapse under the weight of the newly deposited material, thus destabilising the slope.

Weak layers. If thin layers of lower-strength material exist in the sediment, under the right conditions they may act as glide planes and promote failure of the slope.

Seismic activity. Even if they do not cause failure outright, cyclic loads due to earthquakes can disturb sediment structure, increasing pore water pressure and bringing the sediment closer to failure.

Dissociation of gas hydrates. Gas hydrates are solid compounds of natural gas and water that are found in some oceanic sediments and are only stable under particular conditions of high pressure and low temperature. Under increasing temperature or decreasing pressure hydrates may dissociate into gas and water, leaving the host sediment significantly weakened and thus promoting instability.

Slope deformation

Why do some landslides reach high velocities?

Not all terrestrial landslides develop into catastrophic events involving high velocities, however some do. One explanation that has been proposed is frictional heating, i.e. that due to friction at the base of the sliding mass temperature increases and causes expansion of the soil pore water, collapse of the soil skeleton and pressurisation of the slip plane, greatly diminishing frictional resistance and allowing the slide to accelerate more quickly.Better understanding this potential mechanism and the factors that control its severity will enable the future design of appropriate mitigation measures and robust defences.

To achieve this, at Southampton we develop models that describe landslide evolution and use them to gain insights on material properties and insitu conditions that may promote catastrophic failure. In these models we account for the dynamics of the moving mass, the thermo-mechanical behaviour of the soil, the production and diffusion of heat during sliding, the resulting increase of pore water pressure and the flow of water this causes.

Thermal pressurisation

Results show that an important factor is the extent to which a soil is permeable to water. Pressurisation can more easily take place in less permeable soils such as clays, as water finds it more difficult to escape; low permeability therefore facilitates catastrophic collapse. The size of a slide is also important. Taking frictional heating into account leads to the perhaps counter-intuitive result that, all other parameters being equal, larger and thicker slides will accelerate faster than smaller, shallower ones.

Research in this area is ongoing, with the further development of our models to include the initial, sometimes prolonged, phase of slow, creeping movement that many slides exhibit, and the identification of conditions that may cause the transformation of this slow movement into a catastrophic collapse.